1. Field of the Invention and Related Art Statement
The present invention relates to a photometering apparatus comprising a light emitting portion including a light emitting element for projecting a radiation beam to a substance to be measured, and a light receiving portion including a light receiving element for receiving radiation emanating from the substance and converting it into an electric signal.
The above mentioned photometering apparatus has been widely used in various kinds of photometries, particularly in a turbidimeter for measuring the turbidity of a particle suspension and in a colorimeter for measuring the absorbance of a test solution for radiation having a specified wavelength. For instance, in Japanese Patent Application Laid-open Publication, Kokai-Sho 60-259,935 (corresponding to U.S. Pat. No. 4,725,148) there is disclosed a turbidimeter including a semiconductor laser diode serving as the light emitting element and a semiconductor photodiode serving as the light receiving element. Such a turbidimeter is preferably used in the fermentation apparatus for measuring the turbidity of a culture solution. In case of using the semiconductor photodiode as the light receiving element, there is produced a measurement error due to the variation of the operation temperature of the semiconductor photodiode. In order to avoid such a drawback, in FIG. 5 of said publication the temperature compensation is effected by amplifying an output signal of the semiconductor photodiode by an operation amplifier with a negative feedback.
In the turbidimeter having the temperature compensating means disclosed in the above mentioned publication, the turbidity can be measured with the precision of first order below the decimal point, i.e. 10.sup.-1, but can not be measured with much higher precision such as second and third order below the decimal point (10.sup.-2 and 10.sup.-3). In the recent turbidimetry for use in the fermentation apparatus, it is earnestly required to provide the turbidimeter which can measure the turbidity very accurately with the precision of the third order below the decimal point so that the variation of the culture solution within a short time period can be detected precisely.
In order to effect the temperature compensation much more accurately, the inventors of the instant application had devised experimentally a turbidimeter shown in FIG. 1A in which in addition to a first semiconductor photodiode 1 there is provided a second semiconductor photodiode 2 having the same construction as the first semiconductor photodiode, said first and second semiconductor photodiodes 1 and 2 being installed in a single housing 3, while the second semiconductor photodiode 2 being shielded from light, and a difference between output signals from the first and second semiconductor photodiodes 1 and 2 is derived by a differential amplifier 4. In this turbidimeter, since the operational temperatures of the first and second semiconductor photodiodes 1 and 2 are identical with each other, the temperature compensation can be effected by subtracting the dark current of the second semiconductor photodiode 2 from the output signal from the first semiconductor photodiode 1. However, the temperature variation of the differential amplifier 4 for deriving the difference between the output signals from the first and second semiconductor photodiodes could not be compensated sufficiently, so that the output signal from the differential amplifier 4 contains an erroneous component due to the temperature variation. In this manner, the measuring accuracy could not be improved even by using the turbidimeter illustrated in FIG. 1A. Particularly, the temperature drift of the differential amplifier 4 shows the non-linearity and the measurement error increases when the temperature becomes higher.
In order to mitigate the above explained drawback, the inventors further devised a turbidimeter shown in FIG. 1B. In this turbidimeter, the first semiconductor photodiode 1 and the second semiconductor photodiode 2 which is shielded from the external light are arranged in the same housing 3, and the output signals from these semiconductor photodiodes are supplied via signal conductors 5 and 6 to the differential amplifier 4 which is arranged at a position which is remote from the housing 3 and is not affected by the temperature variation. In this turbidimeter, the temperature drift of the semiconductor photodiode can be sufficiently canceled out and the temperature drift of the differential amplifier 4 can be suppressed. However, the long signal conductors 5 and 6 catch noises, and thus S/N of the output signal of the differential amplifier is decreased to an inadmissible extent. Since the impedance of the semiconductor photodiodes is rather high, the signal conductors 5 and 6 are liable to pick-up noises.
2. Summary of the Invention
The present invention has for its object to provide a novel and useful photometering apparatus, in which the temperature drift of the light receiving elements and amplifier can be compensated for sufficiently, while S/N of the output signal can be made high, so that the photometry can be carried out very accurately.
According to the invention, in a photometering apparatus including a light emitting portion having a light emitting element for projecting a radiation beam to a substance to be measured, and a light receiving portion having a first light receiving element for receiving radiation emanating from the substance and converting it into an electric signal, the improvement comprises a second light receiving element which has the same construction as that of the first light receiving element; a housing in which said first and second light receiving elements are installed such that the first light receiving element receives the radiation emanating from the substance, but the second light receiving element does not receive said radiation; first and second operational amplifiers having the same construction and being arranged in said housing, said first and second operational amplifiers being connected to receive and amplify output signals from the first and second light receiving elements to derive first and second amplified output signals, respectively; and a differential amplifier arranged remote from the housing and having first and second input terminals connected to said first and second operational amplifiers and deriving a difference between said amplified first and second output signals as a photometered output signal.